Processes for depositing layers on objects by means of a voltage which pulsates to a greater or lesser extent are known from the prior art. For example, unregulated voltage spikes in the microsecond range are produced by means of thyristor-controlled rectifiers. These voltage spikes are pure interference pulses and are not used as a reproducible method for influencing the deposition result. Furthermore, the following disadvantages are symptomatic of working with poorly smoothed thyristor rectifiers.
1. Spark formation even under the coating surface on the sheet-metal surface to be coated. PA0 2. Severe electrolysis. PA0 3. Film thickness reduction. PA0 4. Formation of flakes in the foam layer and on the sheet-metal edges. PA0 5. After production of a breakdown, a greater reduction in voltage is required in order to reliably avoid this phenomenon with the next part to be coated.
From Brown, William B. (Journal of Paint Technology Vol. 47, No. 605, June 1975), it is known for a square pulse shape in the region of seconds to be produced by interrupting (disconnecting) the deposition current. This procedure has a number of disadvantages. For example, the specified pulse durations are in the region of seconds, preferably up to 3-20 seconds. In these relatively long pauses, on the one hand, the heat is dissipated and, in consequence, the layer resistance is increased. On the other hand, a redissolving effect also occurs, and in addition a softening of the deposited film and removal of gas bubbles as a result of the coating flow. This results in a reduction in the film resistance.
The reduction in the heat developed and in the peak current must in this case take place by slowly raising the voltage. Specifically, if one starts with a pulsed square-wave voltage at the full coating voltage immediately, then the rating of the rectifier must be more than doubled. This increases, in particular, the costs for the rectifier.
Furthermore, the currently available rectifier generators have considerable disadvantages. Specifically, depending on the type, they have a residual ripple which depends on the nature and quality of the rectification and smoothing of the input AC voltage (cf. Vincent, Journal of Coatings Technology Vol. 62, No. 785, June 1990). In addition, this residual ripple is load-dependent, that is to say feedback takes place via the coating process itself. This residual ripple is then also evident only as interference.
From T. Ito and K. Shibuya, Metal Finishing, April 1967, pages 48-57, "Anodic Behavior in Electrophoretic Coating of Aluminum Alloys", it is known for pulsed signals to be produced by alternating current that has been smoothed more or less poorly. Furthermore, processes using alternating-current deposition are known from the German Laid Open Specification 1646130 and the British Patent Application 1376761. In this case, anode plates are used as rectifiers. The anode plates pass current in only one direction, because of special coating.
However, to date, all the described processes have considerable defects. In particular, the breakdown behavior, throwing power, film thickness and film defects are, for example, dependent, inter alia, on the magnitude of the voltage in electro-dipping. In practice, this voltage is normally chosen such that an adequate level of cavity coating is achieved, with the minimum necessary external film thickness, in an acceptable coating time. In order to save coating material, and thus cost, when coating, efforts are made, inter alia, to achieve adequate throwing power with reduced external film thicknesses. With present products and the present technique described above, this development is subject to limits.
The present invention is accordingly based on the object of providing an apparatus for electrochemical coating of objects, by means of which the coating film characteristics and the application characteristics can be influenced systematically in order to obtain, for example, adequate throwing power with reduced external film thicknesses, or in order to achieve preliminary cross-linking during application.
This object is achieved in that an adjustable DC voltage is pulse-modulated by superimposing adjustable AC voltage components on it.
The adjustable AC voltage components are in this case preferably produced from cyclic signals, in particular harmonic oscillations (sinusoidal oscillations), which are easily available.
According to the invention, it is in this case possible by means of suitable circuits to subject the cyclic signals to preprocessing, preferably blocking of the negative voltage elements or rectification.
The invention furthermore provides for the capability to connect and disconnect the superimposition of the AC voltage components on the DC voltage with an adjustable duty ratio. In this way, the pulse modulation, as a variation of the conventional coating process using pure direct current, can be limited to specific time intervals during coating, for example at the start or at the end.
The ranges between 10:1 and 1:10 are known as preferred on:off duty ratios. The duration of the "on" period, in which pulse modulation takes place, is in this case between 10 ms and 100 s.
The DC voltages used according to the invention are in the range from 0 to 500 V. The AC voltage components used for superimposition are likewise between 0 and 500 V. In this case, the superimposition is carried out such that the resultant voltage does not change its direction, that is to say said voltage is a pulse-modulated DC voltage. The apparatus according to the invention is, however, not limited to this, so that it is invariably also possible to operate with a resultant AC voltage, if this provides advantages.
The cycle duration of the cyclic AC voltage components used for superimposition is, according to the invention, between 1 and 500 ms. This corresponds to a frequency of 1000 to 2 Hz. A frequency is preferably used which is obtained from the mains voltage, that is to say, for example, 50 Hz or a multiple of it.
There are various possibilities for producing a pulse-modulated DC voltage according to the invention.
One variant is to connect an AC (variable) transformer in series with a DC generator.
It is likewise possible to couple the AC (variable) transformer via a rectifier, so that a rectified AC voltage is introduced. If a diode is in this case connected between the alternating-current source and the input of the rectifier, further modulation of the voltage is achieved in such a way that only the positive or only the negative half-cycles reach the rectifier.
The optional use of pulse modulation can be carried out such that the AC voltage components are introduced via a mechanical or electronic relay. The latter may be driven via a function generator (that is to say with low current) in order to achieve a defined duty ratio.
A further variant for producing a pulse-modulated DC voltage according to the invention is obtained by connecting a function generator to the phase-gating controller of a three-phase rectifier. This saves the cost and space requirement for an additional AC generator. The function generator may be a commercially available electronic device. It is preferably a programmable microprocessor system, in particular preferably a computer having appropriate software, having an analog/digital converter for receiving the control voltage, and having an output unit for the trigger pulses.
One preferred application of the apparatus according to the invention is for electro-dipping. In this case, the amount of coating deposited in the processing time is directly dependent on the amount of charge which flows--and thus indirectly on the immersion voltage. It must be noticed that a gas layer, which can break down the current flow, occurs at the so-called breakdown voltage, as a result of heating and boiling processes. It is furthermore important to obtain a uniform and adequate film thickness of the coating even at inaccessible points, that is to say an adequate throwing power with reduced external film thicknesses. The process according to the invention surprisingly achieves an optimized result with respect to these requirements, some of which are contradictory.